The present invention generally relates to golf balls, and, more particularly, to a golf ball having a solid core and a multi-layer ionomeric cover.
Golf balls traditionally have been categorized in three different groups, namely, as one-piece, two-piece and three-piece balls. Conventional two-piece golf balls include a solid resilient core having a cover of a different type of material molded thereon. Three-piece golf balls traditionally have included a liquid or solid center, elastomeric winding around the center, and a molded cover. Solid cores of both two and three-piece balls often are made of polybutadiene and the molded covers generally are made of natural balata, synthetic balata, or ionomeric resins.
Ionomeric resins are polymers containing interchain ionic bonding. As a result of their toughness, durability and flight characteristics, various ionomeric resins sold by E. I. DuPont de Nemours and Company under the trademark xe2x80x9cSurlyne(copyright)xe2x80x9d and by the Exxon Corporation (see U.S. Pat. No. 4,911,451) under the trademark xe2x80x9cEscor(copyright)xe2x80x9d and the trade name xe2x80x9clotek,xe2x80x9d have become the materials of choice for the construction of golf ball covers over the traditional xe2x80x9cbalataxe2x80x9d (transpolyisoprene, natural or synthetic) rubbers. The softer balata covers, although exhibiting enhanced playability properties, lack the durability (cut and abrasion resistance, fatigue endurance, etc.) properties required for repetitive play.
Ionomeric resins are generally ionic copolymers of an olefin, such as ethylene, and a metal salt of an unsaturated carboxylic acid, such as acrylic acid, methacrylic acid or maleic acid. Metal ions, such as sodium or zinc, are used to neutralize some portion of the acidic group in the copolymer, resulting in a thermoplastic elastomer exhibiting enhanced properties, i.e., durability, etc., for golf ball cover construction over balata.
While there are currently more than fifty (50) commercial grades of ionomers available from Exxon and DuPont, with a wide range of properties which vary according to the type and amount of metal cations, molecular weight, composition of the base resin (i.e., relative content of ethylene and methacrylic and/or acrylic acid groups) and additive ingredients such as reinforcement agents, etc., a great deal of research continues in order to develop a golf ball cover composition exhibiting the desired combination of playability properties.
Golf balls are typically described in terms of their size, weight, composition, dimple pattern, compression, hardness, durability, spin rate, and coefficient of restitution (COR). One way to measure the COR of a golf ball is to propel the ball at a given speed against a hard massive surface, and to measure its incoming and outgoing velocity. The COR is the ratio of the outgoing velocity to the incoming velocity and is expressed as a decimal between zero and one.
There is no United States Golf Association (USGA) limit on the COR of a golf ball but the initial velocity of the golf ball must not exceed 250xc2x15 ft/second. As a result, the industry goal for initial velocity is 255 ft/second, and the industry strives to maximize the COR without violating this limit. With these considerations in mind, the following objectives of the present invention golf ball are noted.
An object of the present invention is to provide a golf ball having a soft xe2x80x9cfeelxe2x80x9d or compression while maintaining good durability and a high coefficient of restitution.
Another object of the invention is to provide a golf ball having a good coefficient of restitution as compared to other golf balls having a similar compression and surface hardness.
Yet another object of the invention is to provide a soft golf ball having a moderate spin rate and good carrying distance while maintaining a relatively soft compression.
Another object of the invention is to provide a method of making a golf ball having the properties described above.
The present invention provides, in a first aspect, a golf ball comprising a core, an inner cover layer, and an outer cover layer disposed over the inner cover layer wherein the sum of the thicknesses of the inner and outer cover layers is at least 0.090 inches.
The core comprises an improved polybutadiene composition having an ultra-high Mooney viscosity and/or is high molecular weight polybutadiene, and preferably, a polybutadiene formed from a neodymium catalyst.
The inner cover layer comprises a particular combination of components as follows. The inner cover layer comprises at least 50 percent by weight of an ionomer and has a thickness of 0.040-0.050 inches. The resulting inner cover layer exhibits a Shore D hardness of 56-65. The outer cover layer also comprises an ionomer and is harder than the inner cover layer and exhibits a Shore D hardness of 65-71. The outer cover layer has a thickness of about 0.050-0.060 inches.
The resulting golf ball exhibits a PGA compression of about 65-75, and a coefficient of restitution of about 0.786-0.806. The golf ball has a weight in the range of 45.0-46.0 grams. Preferably, the resulting golf ball utilizes a particular configuration of components such that the ratio of the diameter of the ball to the sum of the thickness of the inner and the outer cover layer is less than 19:1.
In yet another aspect, the present invention provides a method of making a multi-layer golf ball comprising obtaining a golf ball core, forming a thermoplastic inner cover layer over the core, and forming an outer cover layer over the inner cover layer. The inner cover layer which is formed exhibits a Shore D hardness of about 56-65 or less. The outer cover layer which is formed over the inner cover layer exhibits a Shore D hardness of about 65-70 and is harder than the inner cover layer. The combined thickness of the inner cover layer and the outer cover layer is at least 0.090 inches.
In still another aspect according to the present invention, a multi-layer golf ball is provided which comprises a core, a mantle or inner cover layer disposed on the core, and an outer cover layer disposed on the mantle layer. The mantle layer comprises at least 50% ionomer based on the weight of the mantle layer. The mantle layer exhibits a thickness in the range of from about 0.040 to about 0.050 inches. The outer cover layer exhibits a hardness which is greater than the hardness of the mantle layer. The sum of the thicknesses of the mantle layer and the outer cover layer is at least 0.090 inches.